The Carbohydrate Nutrition of Tomato Roots

Abstract

A study of certain aspects of the respiratory metabolism of excised tomato roots has been undertaken. Mitochondria derived from such roots possess an active Krebs cycle. Neither the operation of the Krebs cycle nor the glucose-6-phosphate dehydrogenase activity of the preparations are inhibited by mannose. Tracer experiments using mannose-U-¹⁴C indicate that mannose, on entering the root, is rapidly phosphorylated to mannose-6-phosphate which accumulates due to lack of phosphomannose isomerase activity in the tissues. The formation of mannose-6-phosphate is due to the activity of a hexokinase, the presence of which has also been demonstratedIn vitro. The participation of mannose in the hexo-kinase reaction implies its competitive interaction with the natural substrates of this enzyme. Accumulated mannose-6-phosphate probably also inhibits respiration through its demonstrated competitive inhibition of phosphoglucose isomerase. Certain observations suggest that it may also inhibit respiration by depleting the intracellular level of inorganic phosphate. Glucose antagonizes the mannose-inhibition of respiration. One effect of glucose is to inhibit mannose uptake. An enhanced glucose level may also promote the formation of glucose-6-phosphate rather than mannose-6-phosphate by the hexokinase system.